ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. B
4-8
Settings for a solidly grounded system:
I = 0.25 I
N
= 0.25 A
(The setting I = 0.1·I
N
is usually chosen for a impedance
grounded system.)
Stabilising resistor
R
U
I
S
³= =
D
W
60
025
240
.
Chosen R
S
= 300 W
Check of the overvoltage at maximum fault current
A000,100
01.002.0
10003
xx2
I3
I
0syssys
N1
F
=
+
´
=
+
=
IA
2
100 000
1
1000
100==,
,
DU
i
= R
S
I
2
= 300 x 100 = 30,000 V
UUUU
kikmax
()=-22 D
V905,6)200000,30(20022 =-=
Since this value exceeds the permissible maximum peak value
of 2,000 V, a non-linear resistor must be connected across the
circulating current circuit to limit the voltage. Shorting contacts
may also be necessary.
C.t specification:
Rated currents 1000/1 A
Winding resistance R
2
£ 5 W
Knee-point voltage U
k
= 200 V
Magnetising current I
2m
£ 2% I
2N
i.e. I
2m
£ 0.02 A at U
2
= 60 V
The c.t’s must conform to British Standard 3938, Class X.
The c.t’s should also:
· have evenly distributed secondary windings on a toroidal
core (i.e. to minimise the secondary leakage flux)
· not have any winding correction.
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